The present invention relates to safety brake assemblies and, in particular, to safety brake assemblies which permit rotation of a shaft in one direction and inhibit rotation of the shaft in the other opposite direction.
Certain drive systems are subject to torsional stresses which are stored as reactive torque in a drive train. When drive power to the system is interrupted, the reactive torque is released as back-spin and, if an uncontrolled release of torque occurs, personal injury and/or property damage can result. For example, deep well submersible pumps such as progressing cavity pumps driven by sucker rod strings are commonly used to pump oil from deep wells. The drive string for these submersible pumps usually have a relatively small diameter of xc2xe to 1xe2x85x9 inches. Such drive strings are commonly used in wells that vary from 1,500xe2x80x2 to 6,000xe2x80x2 in depth, 3,000xe2x80x2 being a common average. Progressing cavity pumps include a stator which is attached to a production tubing at the bottom of a well and a rotor which is attached to a bottom end of the drive string. Progressing cavity pumps are frequently used to pump viscous crude oil which is often laden with sand or other impurities. As a result, the elongated drive string is subject to considerable torsional force. This torsional force is stored in the elongated drive string as reactive torque. In a 3,000 foot string, as many as several hundreds of revolutions of torsion can be stored in the string if viscous sand laden crude oil is being pumped. When power is interrupted to the drive string, the reactive torque is released. Unless the release of reactive torque is controlled, costly and undesirable damage to equipment and/or personal injury to workmen in the vicinity can result. This is particularly true if an electric motor is used as a power source because such motors offer almost no resistance to reverse rotation.
Using a progressing cavity pump as an example, if drive power is interrupted to the drive string the reactive torque is desirably released in a controlled fashion. A progressing cavity pump may operate at speeds such as from one hundred to five hundred revolutions per minute. Brakes which simply prevent the release of the reactive torque in the drive string are unsatisfactory for two reasons. First, it is preferable that in the case of an electric motor drive, the motor restart unattended when power is restored. In order to ensure a successful unattended restart, the motor must start without load. If the reactive torque in the drive string is not released prior to restart, the motor may not be capable of restarting and the motor may be damaged as a result. Second, if pump repair or replacement is required any unreleased torque in the drive string can be extremely dangerous for unaware workmen. Severe personal injury can result from the unintentional release of reactive torque in such drive strings.
Consequently, braking systems have been invented to controllably release the back-spin due to reactive torque in elongated drive strings. One system is a fluid brake that includes a pump engaged only when reactive torque is released from the drive string. The pump is used to circulate hydraulic fluid or lubricating oil from a reservoir to a bearing case through a restricted orifice. The resistance of the fluid created by the restriction serves to control the release of reactive torque. Nonetheless, this apparatus is subject to the disadvantage that it must be engaged by an over-riding clutch mechanism when back-spin is initiated.
U.S. Pat. No. 5,749,416 describes an over-running or overriding hydraulic clutch to provide braking on the drive string of a downhole pump when power is interrupted. A free-wheel permits the drive shaft to rotate in the desired direction when the pump is running. When power is interrupted, the clutch engages and inhibits the reactive torque in the drive string from rotating the drive string backwards out of control. The problems with an overriding clutch are that overriding clutches are meant to run mainly in an engaged position. They disengage reliably but they do not necessarily engage reliably. Also, an overriding clutch wears when running disengaged. In the case of a progressing cavity pump, the over-riding clutch may be running almost exclusively in a disengaged position. The wear would then be considerable. This means that it could not then be relied upon to engage properly.
Other brake systems for controlling reactive torques have been invented. Those brake systems are based on centrifugal braking principles. Examples of such braking systems are found in U.S. Pat. No. 4,216,848 which issued to Toyohisa Shiomdaira on Aug. 12, 1980; U.S. Pat. No. 4,797,075 which issued to Wallace L. Edwards et al on Jan. 10, 1989 and U.S. Pat. No. 4,993,276 which issued to Wallace L. Edwards on Feb. 19, 1991. The brakes disclosed in these patents all include brake shoes which are mounted within a housing and are therefore difficult to access and maintain. Such brakes require frequent maintenance when they are used to stop the reverse rotation of drive strings in a controlled manner. In addition, they are mechanically complicated and include custom-made moving parts which are costly to manufacture and expensive to keep in inventory.
It is an object of the invention to provide a relatively simple, reliable brake system which is easy to maintain for permitting axial rotation of a shaft in one direction while inhibiting axial rotation of the shaft in the opposite direction.
According to the present invention there is provided a braking assembly for permitting axial rotation of a shaft in a first direction and inhibiting axial rotation of the shaft in a second, opposite direction, comprising: a hydrostatic clutch having a torque generating member and a torque receiving member wherein the torque generating member is connected to the shaft and the torque receiving member is stationary, and a guide means for maintaining the torque generating member in a first de-activated position when the shaft is rotated in the first direction and for moving the torque generating member to a second activated position when the shaft is rotated in the opposite direction whereby activation of the torque generating member inhibits rotation of the shaft in the second direction.
The design of hydrostatic brake components such as the torque generating member and the torque receiving member is known to those skilled in the art. These two components would normally be turbines which include a plurality of vanes circumferentially arranged around the axis of the drive shaft to which the torque generating member is operatively connected. The vanes are angled to provide fluid coupling in one rotational direction about the drive shaft axis.
In this description and in the drawings the invention will be illustrated by its use in progressing cavity pumps. However, it is to be understood that the invention may be used in any system having a drive shaft which needs a braking system for a rotational direction opposite to a working rotational direction.
The guide means may be any suitable means to separate the torque generating member from the torque receiving member when the shaft is rotating in a working direction and which bring the torque generating member and the torque receiving member into fluid coupling connection in an opposite direction.